The present invention relates to a microstructure producing method for forming a minute particle or thin line constructed of a metal or semiconductor, which is minute enough to produce a quantum size effect on an insulative substrate and to a semiconductor device employing a microstructure used as a single-electron device or a quantum-effect device.
Large-scale integrated circuits (LSIs), which have supported the progress of electronics that currently serves as a key industry, have remarkably improved the performances of large capacity, high speed and low power consumption by microstructural development. However, if the device size becomes equal to or smaller than 0.1 .mu.m, then the device presumably encounters a limit on the conventional principle of operation, and this has stimulated energetic researches on a new device based on a new principle of operation. As this new device, there is a device having a microstructure called the quantum dot or quantum thin line of a nanometer size. The quantum dot of the nanometer size has been subjected to energetic researches for application to a single-electron device that employs, in particular, the coulomb blockade phenomenon, together with a variety of quantum-effect devices. The quantum thin line of the nanometer size is expected to be applied to a super-high-speed transistor utilizing the quantum effect.
As a new trend of future electronics, there is a grope for the fusion of an electronic circuit and an optical communication circuit. In such a case, it is indispensable to mount a photoelectric transducer on an LSI substrate, and accordingly, there is necessitated a light-receiving and light-emitting device employing an Si-based material that is the mainstream of LSIs. The light-receiving device has conventionally been put into practical use with the Si-based material. However, with regard to light emission, it has been the accepted view that light emission is not effected since the Si-based IV-group semiconductors have an indirect transition type bandgap. However, it has lately been confirmed that light emission is effected by a minute crystal grain having a size of not greater than 10 nm due to the existence of a direct transition type band structure, and this has stimulated energetic researches.
Aside from the aforementioned example, there have been conducted a variety of researches on the formation techniques of the quantum dot or quantum thin line, intended for application to a variety of electronic and optical devices utilizing the features of the quantum effect and so on. The formation techniques of the quantum dot or quantum thin line disclosed in the following reference documents of (1) through (5) will be described below.
(1) Reference document of Japanese Patent Laid-Open Publication No. HEI 8-64525
FIG. 20 is a sectional view showing the construction of the "Quantum dot producing method and single-electron transistor employing the quantum dot" disclosed in the above reference document of Japanese Patent Laid-Open Publication No. HEI 8-64525. The above single-electron transistor is fabricated by forming an insulating film 72 on a silicon substrate 71, thereafter depositing a conductive film on the insulating film 72 and patterning the conductive film for the formation of a source region 74 and a drain region 75. Next, Si minute particles are deposited to a size of 20 .ANG. at intervals of 20 .ANG. in a high vacuum environment at a temperature of 125.degree. C. by the electron beam evaporation method and then thermally treated at a temperature of 500.degree. C. In this stage, in order to stably grow the Si minute particles with good controllability, the deposition temperature of the silicon substrate 71 is lowered close to the lower limit temperature (about 240.degree. C.) of the Si deposition, thereby depositing amorphous Si minute particles. Thereafter, the Si minute particles are crystallized by heat treatment at a temperature of not lower than the crystallizing temperature (240.degree. C.), thereby forming crystalline Si minute particles 73. Next, a gate insulating film 76 is deposited to a thickness of 40 .ANG. on the insulating film 72, crystalline Si minute particles 73, source region 74 and drain region 75, and a gate electrode 78 is formed on the region of the gate insulating film 76 corresponding to a region between the source region 74 and the drain region 75. This single-electron transistor is used by applying a voltage across the source region 74 and the drain region 75 for the formation of a current between the source region 74 and the drain region 75 via the crystalline Si minute particles 73 and controlling the current by a voltage applied to the gate electrode 78. When no voltage is applied to the gate electrode 78, no current flows due to the coulomb blockade phenomenon produced by the quantum size effect in the crystalline Si minute particles 73. However, if a tunnel resistance between the crystalline Si minute particles 73 is made not greater than the quantum resistance by applying a voltage to the gate electrode 78, then a current flows as a consequence of the breakdown of the coulomb blockade phenomenon.
FIG. 21 is a sectional view showing the construction of the "Light-emitting device employing quantum dot" disclosed in the reference document of Japanese Patent Laid-Open Publication No. HEI 8-64525. As shown in FIG. 21, the light-emitting device is fabricated by forming an insulating film 82 of a thin film (30 .ANG.) on a silicon substrate 81, forming crystalline Si minute particles 83 on the insulating film 82 by a method similar to the single-electron transistor fabricating method, thereafter depositing an insulating film 84 of a thin film (30 .ANG.) on the film and particles and further forming a transparent electrode 85 on the insulating film 84. This light-emitting device emits light by injecting a carrier into the crystalline Si minute particles 83 with a tunnel current formed by applying a voltage across the transparent electrode 85 that serves as an upper electrode and the silicon substrate 81 that serves as a lower electrode.
(2) Ishiguro, et al., Japan Society of Applied Physics lectures in the spring of 1996, lecture No. 28a-PB-5, Proceeding p-798 and lecture No. 26P-ZA-12, Proceeding p-64
FIGS. 22A through 22D are process charts showing the "Method for producing uniform Si quantum thin line on SIMOX substrate utilizing anisotropic etching" disclosed in the above reference document of the item (2).
First, as shown in FIG. 22A, silicon nitride (Si.sub.3 N.sub.4) is deposited on a (100) SIMOX substrate constructed of a silicon substrate 91, an oxide film 92 and an SOI (Silicon On Insulator) film 93, and thereafter patterning is performed to form a silicon nitride film 94.
Next, as shown in FIG. 22B, anisotropic etching is performed using TMAH (Tetra Methyl Ammonium-Hydroxide) with the silicon nitride film 94 used as a mask, thereby forming an SOI film 93a having a (111) plane on the pattern edge.
Next, as shown in FIG. 22C, the (111) plane of the sidewall of the SOI film 93a is selectively oxidized with the silicon nitride film 94 used as a mask, thereby forming an oxide film 95.
Then, as shown in FIG. 22D, the silicon nitride film 94 is removed, and thereafter the anisotropic etching is performed again by TMAH with the oxide film 95 used as a mask, thereby forming an Si quantum thin line 96 having a width of 10 nm and a length of 100 nm. The width of the Si quantum thin line 96 depends on the film thickness of the SOI film 93.
In a quantum thin line MOSFET where the Si quantum thin line 96 is formed as a channel region similar to the single-electron device shown in FIG. 21, coulomb blockade vibration, or the feature of the single-electron phenomenon is observed at room temperature (see FIG. 23). FIG. 23 shows the gate dependency of the drain current of the single-electron device employing the Si quantum thin line, where the horizontal axis represents the gate voltage and the vertical axis represents the drain current.
(3) Goto, et al., Japan Society of Applied Physics lectures in the spring of 1997, lecture No. 28a-T-3, proceeding p-1313
According to the "Method for forming quantum dot of metal material" disclosed in the above reference document of the item (3), by the magnetron sputter clustering method for sputtering Al by DC (Direct Current) discharge (220 V, 0.4 A) of Ar gas (4.times.10.sup.-3 Torr) and clustering Al by He gas filled around there, a spherical aluminum cluster having a diameter of 5 to 500 nm is generated.
(4) Sakurai, et al., Japan Society of Applied Physics lectures in the spring of 1997, lecture No. 30a-PB4, proceeding p-515
According to the "Quantum thin line of metal material" disclosed in the above reference document of the item (4), Al is deposited to a width of 30 .mu.m and a thickness of 8 nm on an SiO.sub.2 insulating substrate, and thereafter, Al located in the region other than the region of the Al thin line is oxidized by means of AFM (Atomic Force Microscope). Specifically, Al is oxidized to become an insulating film by applying a voltage across an AFM probe and Al, and the remaining portion becomes the Al thin line having a width of 20 nm.
(5) Yasuda, et al., the 45th Japan Society of Applied Physics Associate Joint lecture meeting, lecture No. 28a-K-3, Proceeding p-751
According to the "Characteristics and applications of oxide film/nitride film as Si selective growth use mask" disclosed in the above reference document of the item (5), as shown in FIGS. 24A and 24B, an electron beam is applied to a very thin SiO.sub.2 oxide film 202 (having a film thickness of 5 to 20 .ANG.) formed on the surface of a silicon substrate 201 so as to desorb oxygen, thereby transubstantiating the applied portion 203 into Si-rich SiOx. Thereafter, Si is made to selectively grow only on the surface of the transubstantiated applied portion 203, thereby forming an Si thin line 204 (FIG. 24C). In this stage, the Si growth is performed by setting the substrate temperature to 580.degree. C. and using disilane (Si.sub.2 H.sub.6) gas as a material gas.
In order to mount the quantum dot or the quantum thin line which serves as the base of the aforementioned quantum-effect device or single-electron device on a substrate identical to that of an Si-based large-scale integrated circuit that has conventionally been the mainstream, the following problems exist.
According to the "Quantum dot producing method and single-electron transistor and light-emitting device employing the quantum dot" of the aforementioned item (1), the crystalline particles of the very small size generating in the initial deposition stage of electron beam deposition cannot be controlled with regard to their growth position, size and density and are strongly influenced by the surface conditions of surface roughness, impurities and so on. Therefore, it is very difficult to assure the uniformity and reproducibility, for which the method is hard to take effect as a mass-production technique.
The "Method for producing uniform Si quantum thin line on SIMOX substrate utilizing anisotropic etching" of the aforementioned item (2) necessitates a depositing process and a removing process for silicon nitride Si.sub.3 N.sub.4 and an etching process for the Si layer. Therefore, the method, which disadvantageously results in high cost and degraded yields leading to low productivity, is hard to take effect as a realistic mass-production technique.
The "Method for forming quantum dot of metal material" of the aforementioned item (3) utilizes the clustering reaction by means of sputtering and vapor growth. Therefore, it is very difficult to assure the uniformity and reproducibility of the growth position, size and density of the crystalline particles, for which the method is hard to take effect as a mass-production technique.
The "Quantum thin line of metal material" of the aforementioned item (4) necessitates the very special microstructural technique such as AFM. However, there is currently no available apparatus capable of performing the formation in the desired positions throughout the entire surface of the substrate, and there is another problem about how to uniformly form the thin line width with satisfactory reproducibility. In developing a mass-production apparatus, there are many problems about how to manage alignment and how to secure a realistic throughput.
Furthermore, according to the "Characteristics and applications of oxide film/nitride film as Si selective growth use mask" disclosed in the reference document of the aforementioned item (5), the grown Si, which is polycrystal, has a crystallinity inferior to monocrystal, consequently failing in materializing a device having superior characteristics. Furthermore, due to the use of electron beam, there is currently a low productivity, for which the method is hard to take effect as a realistic mass-production technique. Furthermore, the width of the thin line is determined depending on the beam diameter of the electron beam, and it is impossible for the currently available beam diameter to achieve a dimension of not greater than 10 nm required for producing the quantum effect.